Monday, December 30, 2013

VMI Honors 30-year Employees

Every year VMI hosts an employee appreciation luncheon in the month of December.  This year, several people celebrated their 30-year anniversary.  Pictured from left to right, are -
 

Ignacio F., Karen S., Thelma M., Cora P., Linda S., Jeannie B., Kathy L., Nancy C., and VMI's President, Dennis Kemp.

VMI employs over 200 people, and has been in business since 1980.  Since 1990, the technical staff has more than quadrupled.  We've come a long ways in diode manufacturing, and the future is bright.  2013 was a great year, and 2014 looks even brighter!   

Thursday, December 26, 2013

It’s cute as a bug and useful too! The OC025 - 2500V Optocoupler



Manufactured by VMI, the OC025 2500V optocoupler is both small and useful.  

2500V High Voltage Optocoupler from VMI - OC025
OC025 High Voltage Optocoupler

What Makes an Optocoupler?

An optocoupler is made up of a light source, usually one or more LEDs, and at least one photodiode.   
The OC025 uses one photodiode and two LEDs.  The device is encapsulated in an optically clear material, and comes in an amber colored Ultem shell.  It is moisture and solvent resistant. 


The OC025 uses a special, high voltage photodiode as the light-sensitive component.  Designed to be ultra sensitive to light, the photodiode generates leakage current proportional to the light exposure from the LEDs when the photodiode is reversed bias.  


Leakage current from the photodiode is often used as a feedback and control mechanism in instrumentation systems.  Leakage current is also referred to as “reverse current”, and is represented by “Ir”.  “Reverse current” and “leakage current” are often used interchangeably.

High Voltage Optocoupler Operation

The highest levels of leakage current are generated when two conditions are met. 

First, the photodiode must be reversed biased.  Reverse biased, in this case, means the anode voltage is more negative than the cathode voltage, and the device is ‘blocking’.  No forward current is flowing through the photodiode. 


Second, the photodiode junctions must be exposed to light.  The more intense the light, the more leakage current is created.  The greater the range of leakage current, the greater the degree of sensitivity and resolution available to the external control system circuitry.     

Optocoupler Concept of Operation Illustration
High Voltage Optocoupler Concept of Operation
Photodiode leakage current can be adjusted, monitored, and controlled by varying the amount of current through the light-generating LEDs.  Photodiode leakage current can serve as a feedback mechanism for controlling the forward current through the LEDs, which, in turn, determines
the amount of leakage current in the photodiode.
 

Physical Dimensions

Physical dimensions, excluding axial leads, are 0.460” square x 0.325”H (11.7mm x 11.7mm x 8.3mm height).  

Electrical Specifications 

The OC025 is rated at 2500V reverse bias voltage.  In addition,
  •  The DC transfer ratio is 0.76% minimum.
  • Typical T(on) and T(off) are 2us max.
  • The photodiode can dissipate up to 125mW.

For More Info

Contact VMI for more information. 

For the OC025 data sheet or information on other high voltage optocouplers, visit our optocoupler catalog.


***

Thursday, December 19, 2013

From Optodiode to Optocoupler in Three Easy Steps

High Voltage (10kV and 15kV) Optodiodes
10kV OZ100SG and 15kV OZ150SG Optodiode

VMI's 10kV and 15kV optodiodes make it possible to build your own high voltage optocoupler using your choice of LEDs as a light source in three easy steps.
  • Reverse bias the photo-diode
  • Run forward current through one or more LEDs, thus exposing the photo-diode junctions to light
  • Monitor the leakage current, Ir, of the photo-diode.  The level of Ir serves as feedback for the level of forward current through the LED while providing system isolation.  Varying forward current in the LED varies leakage current in the photo-diode.

Applications


Just a few applications include -

  • High Voltage Switches and Remote Control Circuitry
  • HV Linear Regulators
  • Spectroscopy
  • Pockels Cells

Any application that needs high voltage isolation between subsystems, or ground-loop elimination can benefit from the use of optodiodes or optocouplers.

OZ100SG and OZ150SG diodes are 100% tested for Ir, and forward voltage drop, Vf. The overall package is very small - .45x length x .180x x .180x [11.43 mm length x 4.57 mm width x 4.57mm height].

Advantages

Advantages of the OZ100SG and OZ150SG include providing high voltage isolation, and flexibility to your control system in the smallest package available.

Use it as a building block to create your own optocoupler.  

Description of Operation


 The optodiodes represent a breakthrough in opto-diode technology. Capable of operating at up to 10,000V or 15,000V reverse bias (10kV or 15kV Vrwm) the optodiodes are comprised of a minimum of twelve active, stacked, silicon junctions.

The key to operation is light sensitive junctions. Molded in optically clear epoxy, the OZ100SG and OZ150SG are light-sensitive devices. When exposed to light, leakage current is the result. Leakage current is directly proportional to light intensity, so by varying the source's light intensity (such as by varying forward current through multiple LEDs), the leakage current in the photo diode will vary accordingly.  The typical range for leakage current (Ir) is from 1uA to 1nA.

Controlling the exposure to light makes it possible to control leakage current in the photo-diode, thus providing a closed feedback loop. This makes for a useful way to remotely monitor electronically sensitive systems.

Other light sources can be used to generate leakage current in the photo diode.

Factors Affecting Ir in a Photodiode 


Photo-diode Reverse Leakage (Ir) is linear within stated operating parameters.

Leakage current is directly proportional to

  • Level of reverse bias, Vrwm
  • Distance of the light source from the junctions
  • Wavelength of the light source

High Voltage Gain 


When creating an optocoupler using VMI's optodiodes, the High Voltage Gain is defined as
  • Optodiode current divided by LED (light source) forward current x 100%

The optimum light wavelength has been experimentally determined to be
  • Between 890nm and 940nm

Electrical Specifications

 

Electrical specifications for the OZ100SG and OZ150SG include -
  • Vrwm = 10kV (OZ150SG), 15kV (OZ150SG)
  • Io = 50mA
  • Trr = 3000ns

Technical Data Sheets

Dimensions

 

Physical dimensions for both devices are 0.450x length x 0.180x x 0.180xl [11.43 mm length x 4.57 mm width x 4.57mm height]


Pricing 

 

Contact VMI Sales for pricing and delivery.

Monday, December 9, 2013

Not Sure Your High Voltage Diode is Blocking? How to Test It for a Catastrophic Failure.



In manufacturing testing, dedicated high voltage testing equipment is used to check all the important performance characteristics of a diode.  These include Vf, Ir, and Trr.  If you don't have HV test equipment, and suspect your diode is no longer blocking, you can get a ballpark picture of what's going on by monitoring the leakage current when the diode is reversed bias.

Precautions


But first, a few precautions -
  • Consider using a TVS diode to protect your meter
  • Use a current limited HV power supply (approx. 500ua) with current meter
  • Make sure the power is turned off before connecting anything
  • Practice good electrical safety.  Always. 

Okay, now that that's out of the way, the basic process is to apply a reverse voltage to the diode and monitor the leakage current through it.  When a diode loses it's ability to block, leakage current normally increases at a much greater rate as reverse voltage is applied.  The diode is operating in the far (-x, -y) region of the graph below, only the reverse voltage needed to get past the knee of the curve is shifted way to the right, closer to the origin.  Very little reverse voltage generates a large leakage current, relatively speaking. 

Typical Curve Tracer signal for HV Diodes

Testing Steps


The basic testing steps are as follows:

1.  Clean the diode using IPA or a similar solvent (debris can conduct when a reverse voltage is applied and may yield a false-failed reading).

2.  Submerge the diode in clean dielectric fluid such as Fluorinert, or in an inert gas such as SF6.  This is to provide isolation between the leads since isolation through air is about 10kV per inch.  In a diode that is .5", an applied voltage of more than 5kV may cause an arc between leads.  Better safe than sorry.

3.  Using a current limited HV Power Supply (to approx. 500uA) equipped with a current meter,  connect the diode cathode lead to the output of the power supply.  Connect the anode lead to ground. 

Most diodes have a band or dots at one end that indicating the cathode.  If it's unmarked, test it one way, and then flip it around. 

4.  Turn on the power supply and gradually, slowly increase the voltage across the diode while monitoring the leakage current through it.  In a blocking diode, the measured leakage current should be much, much less than 1uA  The nA range is typical.  

If the diode is not blocking, or blocking at a much lower level, the leakage current will be greater than 1uA.  In a multi-junction diode, excess leakage current can increase rapidly around voltage levels greater than 2kV.  (This assumes at least one junction is still operational).  


What's Next

 

If the diode appears to be a dead short, try flipping it around.  If it doesn't block voltage in the other direction, you have a shorted diode.  Throw it away.


That's when you give VMI a call.   
      

Wednesday, December 4, 2013

Design Tips for Getting the Best Performance from Your High Voltage Diodes


VMI's definition of "high voltage" is anything over 1000V.  (Forgive me if I repeat myself). 

The selection process can be intimidating.  There are many diodes to choose from, making it difficult to know where to start.   


Simplify the selection process by keeping the following three things in mind and prevent headaches down the road. 


Keep in Mind the Isolation Between Anode and Cathode

Isolation Between Anode and Cathode Terminations

 

When operating in air, anything over 5kV should probably have added isolation between the anode and cathode leads.  

At low voltages, isolation between the anode and cathode is not usually an issue.  However, if the diode body is .5" (1.27cm) in length, and it's running at 10kV, it is likely to arc in air.  Isolation through air is approx. 10kV/inch (394V/mm), depending on humidity, temperature, and other operating conditions.  At 10kV, the voltage stress on a .5" (1.27cm) long diode is about double what air can support.

Methods to Increase Isolation Voltage Between Terminations

 

There are several ways to increase isolation between the leads.  Over-encapsulation is one method.  If a diode is part of a power supply, many times the entire power supply will be encapsulated.  As long as the encapsulation material adheres to the surface of the diode, it will increase the isolation voltage. 

Conformal coating is a second method.  Parylene conformal coating is common, and is usually done at the pcb or system level.  

Running the part in a vapor, such as SF6, works in some applications.  This requires a sealed environment and is used in specialized applications.  

If air operation is necessary, try connecting several lower voltage diodes in series.  This approach increases the component count and takes up extra space, but it will work in air.  

A second approach to high voltage air operation is to look at a high voltage stick.  A stick uses multi-junction diodes connected in series, which are then encapsulated.  The advantages are greater lead-to-lead isolation and fewer components compared to single junction diodes.   

Design Consideration - Use Hermetically Sealed Diodes in Dielectric Fluid
 

 Running in Oil - Glass or Epoxy?

 

When running in an oil or liquid environment, a glass body diode is recommended. Glass body diodes are hermetically sealed and operate quite well in an isolating fluid such as dielectric oil. 
While epoxy diodes like the K-bodies can handle higher currents, they are not hermetically sealed.  Over time, any fluid may penetrate the epoxy body or wick up through the leads, ultimately causing the diode to fail.

K-bodies will work in a gas, such as SF6, but are not recommended for fluid operation.

Design Consideration - Keeping Your Diodes Cool for Ultimate Performance


How Hot is HOT? -Thermal Considerations 

      Note:  Thermal considerations are more relevant in higher current, high voltage applications but should be given some thought even in lower current apps.


High voltage can mean high reverse power dissipation.  In contrast to low voltage applications, leakage currents can be higher too.  Since dissipated reverse power - in the form of heat - is a function of reverse voltage, reverse current, and reverse recovery time, it makes sense that reverse power losses can no longer be ignored. 
 
Two things happen when a silicon diode heats up.  First, the reverse recovery time (Trr) slows down.  That means the diode sees a longer interval of reverse voltage and reverse current before switching to forward conducting mode.  The product of reverse voltage and reverse current is 'reverse power'.  


As reverse power goes up, so does the temperature of the diode, which means it slows down, which means the dissipation interval gets longer which means the diode heats up which means.....this cycle will continue to escalate until eventually the diode fails due to thermal runaway.  The diode is unable to block.  It will conduct in the forward and reverse directions.    

Secondly, for a given forward current (Io) the forward voltage (Vf) will decrease.  As silicon heats up it becomes less resistant until eventually it will act as a short.  A very hot diode with a lower resistance means it will draw more current.  More current means it heats up more, which lowers Vf which means it draws more current.....As you can see, dissipated power - both forward and reverse - become significant.  

But not to worry! 


What to Do About Hot Diodes 

 

If it turns out that your diodes dissipate high reverse power in your app, there are a couple of things you can do.  First, pick a diode that has a faster recovery time.  The faster Trr limits the amount of dissipated reverse power by reducing the length of time the diode takes to recover and transition to a forward conduction state.

Secondly, de-rate, de-rate, de-rate.  If the diode is only idling as opposed to running full blast, it will stay cooler.  A cooler diode is a happy diode.  


If your app requires 50mA, but the operating temp is 100C, you might consider using a diode rated for 500mA.   

Third,  add heat sinks or heat spreaders.  This can be done by soldering heat sinks to the diode leads, or near the diode body.  If the heat sink is conductive, like copper or aluminum, care must be taken to prevent arcing through the material from the diode leads to the heatsink.

Fourth, forced air, water cooled baseplates, or circulating liquid are excellent techniques for getting the heat out of a diode. 


Lower Power Applications



When selecting a high voltage diode, it's important to keep isolation through air, hermeticity, and thermal dissipation in mind.  Thermal considerations are more relevant to higher power applications, but can pop up in lower voltage apps.     

Selecting the right diode can save you time, money, and effort.  If you're not sure which one to choose, give us a call.  We can help you make the best selection.